make more use of BaseQtVersion::qmakeProperty()

... instead of peeking into versionInfo().
in fact, make versionInfo() private, to avoid subsequent deterioration
as happened before (after 5e596e89d).
some other functions that use it in its interface also become private.

this fixes several breakages related to working with non-installed
prefix builds of qt.

Change-Id: Ib67de79323c9e38f3de48a09854b155de9eef5b2
Reviewed-by: James McDonnell <jmcdonnell@blackberry.com>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: BogDan Vatra <bogdan@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/plugins/android/androiddebugsupport.cpp

@@ -131,7 +131,7 @@ RunControl *AndroidDebugSupport::createDebugRunControl(AndroidRunConfiguration *
         Kit *kit = target->kit();
         QtSupport::BaseQtVersion *version = QtSupport::QtKitInformation::qtVersion(kit);
         if (version) {
-            const QString qmlQtDir = version->versionInfo().value(QLatin1String("QT_INSTALL_QML"));
+            const QString qmlQtDir = version->qmakeProperty("QT_INSTALL_QML");
             params.additionalSearchDirectories = QStringList(qmlQtDir);
         }
     }